Communication systems typically include a plurality of communication units, such as mobile or portable radio units and dispatch consoles that are located at multiple sites. The various sites usually include base site repeaters ("repeaters") for transceiving information such as control, voice, data and network management traffic between the communication units and each other. The communication units are often logically divided into various subgroups, known as talkgroups, which can be made up of communication units at different sites desiring to participate in a group or dispatch call. A dispatch call is one in which members of a particular talkgroup can communication with each other via communication links established between multiple endpoints, such as voice repeaters and dispatch console positions. Dispatch calls are often prioritized according to the source of the communication. For example, a message from a console operator typically receives a higher priority than one from a mobile or portable radio unit.
Next generation radio systems propose to employ multicast addressing protocols such as multicast Internet Protocol (IP) for providing group or dispatch call services. Examples include U.S. patent application Ser. No. 09/283,121, titled "Wireless Communication System Incorporating Multicast Addressing and Method For Use" and U.S. patent application Ser. No. 09/464,269, titled "Methods for Implementing a Talk group Call in a Multicast IP Network," each assigned to Motorola, Inc. and incorporated herein by reference in their entirety. Generally, IP multicasting protocols provide one-to-many or many-to-many communications capability in a connectionless packet network. The network defines a spanning tree of router interfaces and necessary routes between those interfaces to provide multicast distribution of data with a minimum amount of data replication. Moreover, with multicast routing protocols, there is no need for dedicated bandwidth to each endpoint, thus dispatch service can be provided relatively more efficiently and less costly than in traditional circuit-switched networks. However, no solutions are currently known to exist for voice priority among competing sources in an IP multicast network.
Accordingly, there is a need for a method of implementing talkgroup calls in an IP multicast network among competing sources that may have different transmit priorities. The transmit priorities may relate to the priority of the sources per se (e.g., dispatch console having a higher priority than a subscriber unit), the priority of different units of the same type (e.g., police radio having a higher priority than a dogcatcher radio) or the priority of the messages from the respective sources (e.g., emergency calls having a higher priority than status updates). To that end, there is a need for a system and method that allows a source having a higher transmit priority (e.g., dispatch console) to "takeover" an active call from a source having a lower transmit priority (e.g., subscriber radio unit). Preferably, a first group of communication devices in the talkgroup (e.g., monitoring console positions) will be able to hear both the high transmit priority and low transmit priority sources, while a second group of communication devices in the talkgroup (e.g., subscriber units) should hear the high transmit priority source instead of the low transmit priority source after the takeover.
Conversely, there is a need for accommodating a request to transmit ("attempted takeover") from a lower transmit priority source during an active call from a higher transmit priority source. In this scenario, a first group of communication devices in the talkgroup (e.g., consoles) will preferably be able to hear both the high transmit priority and low transmit priority sources, while a second group of communication devices in the talkgroup (e.g., subscriber units) should continue hearing the high transmit priority source and not hear the lower transmit priority source during the attempted takeover. In either the "takeover" or "attempted takeover" situation, there is a need for accommodating cases when the lower transmit priority source dekeys before the higher transmit priority source and when the higher transmit priority source dekeys before the lower transmit priority source. The present invention is directed to addressing these needs.